Heretofore, the amount of fiber suspension per unit of time passing through a conduit, for example a conduit supplying fiber stock to a paper-making machine, has been measured using a fiber concentration meter, and the measured values from this meter have been used for regulating the fiber concentration in the fiber suspension. However, these fiber concentration meters do not have sufficient accuracy of measurement to provide a sufficiently high degree of regulation of the fiber concentration.
The present invention aims to provide an apparatus of the kind referred to which has greater accuracy than the hitherto known apparatus employing fiber concentration means. The invention is based on the realisation that there is a relationship between the fiber concentration of a fiber suspension being pumped through a conduit and the pressure head loss undergone by the fiber suspension in passing through the conduit, after making any necessary allowance for static pressure head loss in the conduit. Thus, for pumping fiber suspensions having concentrations of fiber in excess of a certain minimum value, for example in excess of 2 percent by weight of fiber, the dynamic pressure head loss measured over a section of the conduit through which the fiber suspension is being pumped, is almost proportional to the fiber concentration in the fiber suspension.